JP-A-1-317007 discloses a bus interface communication circuit for communicating by using a differential voltage signal between communication lines. When such a communication circuit is mounted in a vehicle, it is desired to design it as IC. However, when an in-vehicle LAN is constructed by using the communication circuit concerned, particularly when a safety system network containing a vehicle air bag system or the like is constructed, countermeasures for the case in which a communication line is short-circuited to a power supply line for the vehicle or a ground line are required.
JP-A-2000-269801 discloses an interface circuit in which integrated circuits are connected to each other through a transmission line and transmission/reception of data signals is carried out through one transmission line. An impedance device designed so that the terminal impedance thereof is changeable is provided between the transmission line and the power source. This impedance device acts to suppress occurrence of additive DC current.
When circuit blocks having different power supply systems are connected to each other through a transmission line, the potential relationship between both the power supply systems is varied by variation of the power supply systems due to external noise received, short-circuiting of the power supply lines to other potential sites or the like, thereby causing the transmission signals to be erroneously transmitted/received. Particularly when both the circuit blocks are different in ground potential and insulated from each other, the potential relationship therebetween is not settled, and thus it is impossible to transmit/receive the transmission signals normally.